The invention relates to a coupling device for connecting a light source to optical fiber comprising a semiconductor light source, which emits light substantially in accordance with Lambert's law and which is mounted on a holder, and a coupling rod, which is formed by a part of an optical fiber having a circularly cylindrical core and cladding whose refractive index is smaller than the refractive index of the core, the coupling rod forming an optical connection between the light source and a bundle of optical fiber to be connected to the device.
In the field of telecommunications it is known to replace coaxial electrical connections, wire conductors between electronic apparatus with optical connections, optical fibers, between a light source and a light-sensitive receiver.
The optical fiber for use at high frequencies, in the range of gigahertz, offer the advantage that they are not susceptible to external parasitic interference such as cross-talk. Moreover, the attenuation coefficient of these optical fiber is smaller than that of conventional coaxial cables. Finally, optical are electrically insulating and they are also lighter than coaxial cables.
Generally, an optical fiber is formed by a first material in the form of a core which has a refractive index N.sub.1 and which is covered by a second material in the form of cladding having a refractive index N.sub.2. The refractive index N.sub.2 is smaller than the refractive index N.sub.1. The propagation of light inside the fiber core results from the difference between the refractive indices N.sub.1 and N.sub.2 of the core and the cladding, respectively.
For particular transmission applications, use may be made of only a single fiber. However, for reasons of reliability, several optical fibers are typically used for many signal transmission applications. These fibers are grouped in the form of a bundle of 7 or 19 fibres inside a jacket.
The use of a single fiber or of a bundle of several optical fibers, however, gives rise to some problems, notably in their connection to a light source.
It is known that the angle of radiation incidence of an optical fiber may not be larger than 10.degree.. As a result, if the light source itself is of a substantially concentrated nature, for example, when the source is a laser, the connection of this source to the can be realized with high efficiency. However, if the light source is a semiconductor diode, which generates an emission of light at the semiconductor junction, the efficiency of the coupling to the fiber is substantially less.
In practice an electroluminescent diode is used as the light source in many cases. On the surface of a semiconductor crystal, light is generated which is emitted from the surface either directly or after having passed through the diode material. The electroluminescent diode in both cases acts as a spatial light source which emits light in accordance with Lambert's law, with the result that only a small amount of light emitted by the diode can be intercepted and conducted by the fiber. Therefore, in order to ensure optimum data transmission in a fiber or a bundle of fibers, it is desirable to radiate a maximum quantity of light into the fiber or the bundle of fibers. At the same time, alignment and the spacing of the light source with respect to the entrance face of the fiber or the bundle of fibers (this face is sometimes referred to as the "receive face") must be accurately performed and checked. Obviously, the more emission of the light source satisfies Lambert's law, the more important it is that these conditions are satisfied.
In order to increase the amount of transmitted light and to ensure correct transmission, it is known to use a bundle of 7 or 19 fibers, so that a receive face having a large cross-section is obtained. An optical for example a lens, is system added to the bundle in front of the receive face.
It is thus possible to intercept the radiation from the source at an angle which is larger than the maximum angle of radiation incidence of the fiber, and to refract the radiation from the source subsequently to an angle which at most equals maximum angle of incidence, with the result that the fiber (fibers) can receive and conduct the light.
In accordance with the described technique, the formation and the arrangement of the optical system must be realized with the utmost of care and with extreme precision. These requirements which give rise to a high cost for the optical device thus obtained.
A further coupler is disclosed in French Pat. No. 2,075,117. According to this patent, the electroluminescent diode and the receive face of the bundle of fibers are covered with a mass of transparent material having a refractive index which is larger than 1 in order to increase the quantity of light emerging from the diode.
This coupler has a number of drawbacks, notably regarding the alignment of the centers of the light source and the receive face of the optical fiber or the bundle of optical fibers.
The most recently proposed coupler consists of an intermediate element which is introduced between the light source and the bundle of fibres; this element is referred to as a "coupling rod" and is formed by a part of an optical fibers which, as is known, comprises a core and cladding of two materials of different refractive index.
This coupler, which can also be used between a bundle of fibers and a light-sensitive receiver, offers the advantage that the centering of the relevant elements with respect to each other is facilitated. When this is used in the most common case, i.e. for coupling a light source and a bundle of fibers, the cross-sectional area of the coupling rod must be larger than the cross-sectional area of the light source, but smaller than the overall cross-sectional area of the bundle of fiber.
The centering with respect to each other and the fixation of the light source and the coupling rod in the correct position is provided by a support which is constructed in known manner so that it exerts, via suitable mechanical means, a radial force on the coupling rod and/or on the light source.
Taking into account the most recent progress in mechanics, it has been found that, with respect to sources emitting radiation substantially in accordance with Lambert's law, the most important factors for obtaining efficient optical transmission are the cross-section of the light source and the distance between the source and the bundle of fibers or the distance between the source and the coupling rod.